Casein is any of a group of phosphate-containing proteins (phosphoproteins) isolated from milk. It contains 0.71% phosphorus, 0.72% sulfur, and about 15 amino acids. It constitutes about 80% of the protein in cow's milk and about 40% in human milk.

Casein is precipitated out of skimmed milk in acid conditions or by the action of rennin; it is readily digested by the enzymes of young mammals and represents a major source of phosphorus. Casein is very easily prepared and is useful as a protein supplement, particularly in the treatment of malnutrition. In alkaline solution, casein forms a colloid used as a glue, a binder for paint pigments and paper coatings, and to dress leather. Casein is also used to make plastics and cosmetics.